He can't hold onto Star Wars forever. Eventually he'll die, and his estate will screw it up anyway. It's better to let go of it now, when he's still alive and can influence the trajectory, rather than not know where it's going to end up later.
Disney is a natural fit for Star Wars. It has always been about the children. That's why there were Ewoks. That's why there was Jar Jar. It's always been the sort of movie that Lucas himself wanted to see when he was a kid.
Then there is the problem with copyright. Lucas believes that movies are dead in the future. Cheap 1080p systems for the home already rival movie theaters in picture and sound, and the Internet makes piracy too easy. The copyright genie will never be put back in the bottle. Again, Disney is a natural fit. They are not a film company, they are a force of nature. In the future, film companies will die off, but Disney will survive.
Disney will do no worse than what Lucas himself did with the prequels. Star Wars is a brand. Disney will do the do the sorts of things that companies do for brand management. They'll produce content that is critically regarded, loved by fans, but a commercial failure. Then they'll also produce content that's the opposite, milking the brand for money. We'll get to complain and fall in love again with the franchise all over again many times in the coming decades.
So yea, it's the end of the era, but the start of a new one.